1. Field of the Invention
The present invention relates to a charging system, a process cartridge and an image forming apparatus, in which charging is executed with conductive particles.
2. Description of the Related Art
In an image recording apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a corona charger has been employed as a charging apparatus for a uniform charging process (including charge elimination) to obtain a desired polarity and potential of an image bearing member (member to be charged) such as an electrophotographic photosensitive member or an electrostatic recording dielectric member.
Recently there is also in use a charging apparatus of a contact method (contact charging apparatus) which executes charging by contacting a charging member, to which a voltage is applied, with the member to be charged. However, even in such contact charging apparatus, drawbacks resulting from by products of the discharge, such as ozone, are basically not avoidable.
In order to avoid such drawbacks, there has been proposed an injection charging method in which a charge is directly injected from a contact charging member into the member to be charged. For example, there is utilized a mechanism of direct charge injection by applying a voltage to a contact charging member such as a charging magnetic brush, and injecting a charge into a trap level originally present in a surface of the member to be charged or into a trap level arbitrarily prepared by forming a charge injection layer including a charge retaining material such as conductive particles (Japanese Patent Application Laid-open No. 06-003921).
In contrast to the known contact charging which is principally based on a discharge phenomenon and requires an application of a voltage equal to or higher than a discharge threshold value, the above-mentioned injection charging allows to charge the member to be charged to a potential corresponding to an applied voltage, even below the discharge threshold value.
However, in the injection charging method employing a magnetic brush or the like as employed in the aforementioned Japanese Patent Application Laid-open No. 06-003921, a mechanism for holding the charged magnetic particles and maintaining such charged magnetic particles in sliding contact with the member to be charged is a factor of an increase in the cost.
Therefore, for a simpler injection charging method, there is proposed, as disclosed in Japanese Patent Application Laid-open No. 10-307454 etc., a configuration in which conductive fine particles (conductive particles) for improving the contact charging property are made present on a flexible roller constituting a contact member. In such configuration, the contact between the contact charging member and the member to be charged can be controlled by an amount or a size of the present conductive particles and satisfactory charging ability can be obtained.
Also the contact charging method employing conductive particles is advantageously employed in a cleanerless system, in which there is no mechanism for removing a developer remaining on the member to be charged after a transfer step, and there is proposed, as shown in Japanese Patent Application Laid-open No. 2000-081760, a configuration of mixing the conductive particles in the developer thereby maintaining the amount of the conductive particles on the contact charging member and sustaining the charging ability thereof.
In the aforementioned contact injection charging method utilizing the conductive particles, it is already known that the charging condition is determined by mutual correlation of various parameters, but the correlation of individual parameter is not clarified and it has been difficult to predict an applicable range for each individual parameter under various situations.
For this reason, the charging state shows significant fluctuation in the stability under environmental conditions or in durability, creating an obstacle in the system designing.
In particular the injection charging characteristics are influenced also by the electrical characteristics of the member to be charged, and it has been desired for example to realize an appropriate surface resisvisity on the member to be charged.
An object of the present invention is to provide a charging system, a process cartridge and an image forming apparatus capable, in a charging method utilizing conductive particles, of realizing appropriate various conditions relating to the charging.
Another object of the present invention is to provide a charging system, a process cartridge and an image forming apparatus suitable for an injection charging method.
Still another object of the present invention is to provide a charging system, a process cartridge and an image forming apparatus in which electrical characteristics of a member to be charged are made appropriate.
Still another object of the present invention is to provide a charging system, a process cartridge and an image forming apparatus with a high charging stability.
Still other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description, which is to be taken in conjunction with the accompanying drawings.